A molten aluminum alloy has a property of reacting with a metal, such as iron, to produce an intermetallic compound. Those steel parts of a casting machine which are in direct contact with a molten aluminum alloy can be damaged due to their reaction with aluminum. This phenomenon is called melting loss. In casting of an aluminum alloy, it is essential to take measures against melting loss for chief parts, such as a conduit, a mold, a sleeve and an insert, which are to contact a molten aluminum alloy.
A steel material, such as a tool steel which has undergone a nitriding treatment, is generally used for a mold, etc. for use in aluminum casting. The nitriding treatment, which comprises diffusing nitrogen from a steel surface to form a hard nitride layer, is excellent in enhancing the wear resistance of the material. It has been pointed out, however, that such treatment is not always sufficient for preventing a melting loss.
With respect to members for which high melting loss resistance is required, it is a common practice to form a ceramic coating on the surface of a member by a vapor deposition method, such as PVD (physical vapor deposition) or CVD (chemical vapor deposition). Such a ceramic coating is known to be chemically stable to a molten aluminum alloy and exhibit very high melting loss resistance (see New Mechanical Engineering. Handbook, B2, Processing/Processing Devices, p. 157).
The biggest problem with a ceramic coating, as formed by PVD or CVD, is peeling due to a thermal stress. In particular, because of a large difference in thermal expansion coefficient between a steel base and a ceramic coating, a large thermal stress will be produced at the boundary between the ceramic coating and the steel base by the repetition of heating and cooling during successive casting cycles. The large thermal stress often causes peeling of the ceramic coating from the base, whereby the base comes into direct contact with a molten aluminum alloy. Melting of the steel base thus begins suddenly, resulting in a melting loss of the base.
For the purpose of preventing such peeling of ceramic coating, various improvements have, been made in methods for forming ceramic coatings in order to reduce the thickness of a coating, thereby minimizing a thermal stress generated at the boundary between the coating and a base, or to enhance the bonding strength between a coating and a base.
Despite the various improvements, however, the fundamental difference in thermal expansion between a ceramic coating and a steel base has been an insurmountable bar, and complete prevention of peeling of a ceramic coating has not been achieved as yet.
It is therefore an object of the present invention to solve the above problems in the prior art and provide a metal material for parts of a casting machine and a molten aluminum alloy-contact member, which have materially enhanced melting loss resistance, without resorting to conventional techniques, such as the provision of a ceramic coating by PVD or CVD.
It is another object of the present invention to provide a method for producing a molten aluminum alloy-contact member, which makes it possible to strongly bond TiC particles to a Ni alloy layer of the member so that the member has materially enhanced melting loss resistance.